1. Field of the Disclosure
The present invention relates in general to disk drives and, in particular, to a system, method and apparatus for sequences of data track usage to reduce the effect of adjacent track interference.
2. Description of the Related Art
Increased magnetic disk recording density has been accompanied by further narrowing of the track pitch. When data is recorded on a particular data track of a magnetic hard disk drive (HDD), a phenomenon in which magnetic flux that has leaked from the recording head (e.g., a fringe field) affects the data that has been recorded on adjacent data tracks and far tracks. When the recorded data has been updated many times, the data that has been recorded on the adjacent data tracks and far tracks can be corrupted and have errors. Errors that can be recovered from error correction codes (ECC) are called soft read errors. However, readback of these tracks may not be recoverable from the ECC.
For adjacent tracks, this non-recovery phenomenon is known as adjacent track interference (ATI). For far tracks, this phenomenon is known as far track erasure or far track interference (FTI). Depending on the design of the write head, both ATI/FTI-refresh and FTI effects (ATI/FTI) occur during the write of a particular data track. Manufacturing tolerances for the write head and magnetic media may affect the ATI/FTI impact on the various head-media interfaces in a single HDD.
The write head is mounted to a rotary actuator such that its movement is arcuate in shape from the inner diameter of the disk to the outer diameter of the disk. Thus, geometry also may alter the impact of ATI/FTI. Since the head moves above the magnetic disk in accordance with the tracks, a skew angle is defined between a tangent of a data track on the disk and the direction of the head.
After data writing has been performed a certain number of times on an adjacent data track, ATI/FTI may be prevented by a process of reading and write-back of the data track. This process is referred to as ATI/FTI-Refresh. To determine when an ATI/FTI-Refresh is to be performed, a Risk-Counter may be used to indicate the extent of the ATI/FTI degradation for each data track. The Risk-Counter may be updated in response to the writes of adjacent or far data tracks. When the Risk-Counter of a data track exceeds a Risk-Threshold, the ATI/FTI-Refresh is performed on that data track.
User data may be written on the disk in data blocks of, for example, 512 bytes or 4 kilobytes. These data blocks employ data encoding techniques and ECC to recover data that, when read back, includes bits in error. The data encoding and ECC have limitations in the number of bit errors that can be corrected by the electronics of the HDD. If there are too many bits in error, not all data bits can be recovered and the data block is considered unrecoverable. Thus, if the ATI/FTI degradation is too far along then the data block in a data track may be unrecoverable. The ATI/FTI-Refresh is intended to alert the HDD electronics before the degradation has progressed too far. When the Risk-Threshold has been exceeded for a data track, that data track is read back, any soft data errors are corrected using the data encoding and ECC, then the corrected user data is written back to the disk at the same track location.
Because the relevant data is read out of the entire circumference of the data track and rewritten on the data track in an ATI/FTI-Refresh, commands from the host computer to which the HDD is coupled cannot be processed during that period. Consequently, the performance of the HDD is degraded during these periods.